Reactance compensation system



fPatenteH Apr-f6 1948 ,REACTANCE COMPENSATION SYSTEM; j

, George H. Brown, Princeton, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application February 14, 1945, Serial No. 511,912 I 8 Claims. Cl. 17844) This invention relates to reactance compensation and more particularly to improvements in the art of neutralizing the effects of series reactances in radio frequency power circuits. As is well known to those skilled in the art, the frequency band width throughout which a radio frequency transmission circuit operates efficiently to transfer power is an inverse function of the series reactances present in the circuit.

The principal object of the present invention is to provide improved methods of and means for compensating the efiects of such series reactances.

Another object is to provide improved methods of and means for obtaining of radio frequency power circuits.

A further object is to provide an improved rotatable joint structure-for-radio frequency transbroad band operation frequency at which the line 1 ,is one quarter wavelengthlong, a very low impedance is presented by it inseries with the load I. At lower frequencies, the line 1 acts like a capacitance connected in series with the load I, while at higher frequencies the line 1 presents an inductive reactance in series with the load I. As long as the reactance of the line 1 is very much lower in magnitude than the impedance Z0 of the load I, ,there is substantially no impedance mismatch-caused by the presence of the line 1. However, at, frequencies outside a certain relatively narrow band, the reactance of the line 1 will be of thesame order of magnitude as the impedance Zn, or higher. At any of these frequencies, a majorlportion of the voltage of the source' 3 will appear across the line 1 rather mission systems, wherein series reactances produced by overlapping coaxial conductors are effectively neutralized throughout a wide frequency band. 1

These and other objects will become apparent to those skilled in the art upon consideration of the following description, with reference to the accompanying drawing, of which Figure 1 is a schematic diagram of a power transmission circuit embodying the invention,

Figure 2 is a group of graphs illustrating the effects of varying the characteristic impedance of one of the transmission line elements in the circuit of Figure 1,

Figure 3 is a further group of graphs illustrating the design of the circuit of Figure 1,

Figure 4 is a schematic diagram of a rotatable joint in accordance with prior art practice, and

Figure 5 is a schematic diagram of a rotatable joint constructed in accordance with the present invention. a

Referring to Figure 1, it is assumed that a load I, of impedance Z0 is to be energized from a source 3, also of impedance Zo, through a transmission line 5 having a characteristicimpedance 20. In the absence of any further circuit elements, the load I and line 5 will be matched to the source 3 at all frequencies, providing efiicient power transfer independently of frequency! It is assumed, however, that it is necessary to connect in series with the load a reactance element, represented in the present illustration by a transmission line section 1, open clrcuited at its free end.

penacircuited transmission lines exhibit, at certain frequencies, characteristics similar to those of series resonant circuits. Thus,- at the than the load Learising serious impedance mismatch and preventing efiicient transfer of energyl.

No great amount of energy is dissipated in the line 1 under the foregoing conditions. The current is, reflected backdown theline 5 to the source 3. Denoting the current'transmitted by v the source 3 as i1, and the reflected current as 2'2, the absolute value of is the reflection coeflicient K. The degree of impedance match is commonly expressed in terms of standing wave ratio R where t Y Referring to Figure 2, the standing wave ratio on the line 5 is a function of frequency, when supplying the load shunted by the line 1 without compensation, as represented by the solid curve A. The ratio ,f/fo represents the departure from the resonant frequency of the line 1, that is it is the resonant frequency of the line 1 and j is the actual frequency of the energy being transmitted. It is seen that at resonance the standing wave ratio is unity, indicating that no reflection occurs. At all other frequencies, R is less than 1, increasing with the departure from ft. 1 The curve of A as well as the other curves ,of Figure;2 ,i sbased on the assumption that the characteristic impedance of the line 1 is one-half the impedance of the load I In accordance with the present invention, the line 5 terminates in a line section ,9 of;one=half wavelength at the resonant frequency of the line 1, and of impedance Zc. A quarter wave line H,

electrically identical with the line 1, is connected across the junction between the lines 5 and 9. It is found that by adjusting the surge impedance Zc of the line section 9 to the proper value, the standing wave ratio may be maintained substantially equal to 1 over a wide range of frequencies. The curves B, C and D of Figure 2 show how the ratio R varies with frequency with values of 7 i A Z he respectively for the surge impedance Z0 of the line section 9. It is seen that with the standing wave ratio remains higher than .9

the ratio It remains substantially constant at unity from .75fo to 1.25fu. The preferred value for Z0 probably lies somewhere between.

1.1 1.2 7 providing a standing wave characteristic interand Z Z, Z Z

[sin (-g-) tan ,s cos tan p tan [cos tan +sin sin Z.Zc z. Z 2. z

Denoting the electrical length of the line section 9 as and that of the lines 1 and II as p/2, the impedance looking toward the load from the point a in Figure 1 is;

(1) a= 0 .7 s cot p/2 0[11- cot (M2)] The impedance looking into the line section 9 from the point I) is:

The impedance looking into the line section 9 through the series-connected line section I l is:

(3) Zi -Zb-':I'Z8 cot /2) Substituting Equation (1) in Equation (2), and Equation (2) in Equation (3):

Separating the real and imaginary components of Equation (5) s V I Q 7 -i.[tan z gc(gp o tan p+Sin (5 s Z Z Z Z,

mediate those shown by curves Band C} Curve This has been done, and the results are illustrated by the curve E of Figure 3. The abscissa of the graph of Figure 3 is a where Z5 is the surge impedance of the quarter Wave line sections 1 and l I, and Z0 the impedance of the load I. From the curve E the proper value of the impedance Zc in terms of the load, impedance Zomay be selected. "Although the lines 5,

l, H and 9 are illustrated schematically in Figure l as parallel open-Wire lines, it is to be understood that any or all of the above lines may be of coaxial construction.

cos tan p I sin g)] tan ,5 sin 5-) i T ZOZS ZZCZZ,

Z o 0 Z a tan The first term is R1 and the second term is Xi. By Equation (6), the values of Z1, R1 and Xi may' be computed for any values of Z5, Z0 and Z0.

The standing Wave ratio may be calculated as mentioned above from the direct and reflected currents i1 and i2.

where A is a constant. The curves of Figure 2 are calculated by substituting the indicated values of Z0, Z0 and Z5 in the above Equations (6), (7) and (8) todetermine the currents i1 and i2 for computing the standing wave ratios.

By differentiating the expression for X1 in Equation (6) with respect to p, substituting p=1r radians, setting 9 =0 and solving for Zr Z Z 1+ /1+4(g Y Z i This represents thepvalufe oi which provides Zerorate of change of X1 with frequency. Equation (9) is represented graphi cally by the line F of Figure 3. It is seen that the curve F deviates from the curve E by a varying amount, up to about 10 percent at This is accounted for by the fact that thestanding wave ratio curve which is flatte'st at f0, where p=1r, is not necessarily the flattest throughout an appreciable range. Since the values of represented by the curve E were determined by inspection of families of curves like that of Figure 2, it is probable that the optimum value for for a given set of conditions, lies somewhere between that indicated by the curves E and F of Figure 3. i

The above-described method of reactance cancellation may be applied to the design of a rotat able joint for radio frequency lines. Referring, to Figure 4, a rotatable coaxial joint constructed in accordance with prior art practice comprises two concentric line sections 2| and 23 arranged so that their inner conductors and outer conductors respectively overlap by a quarter wavelength, at the mean operating frequency. The diameter of the outer conductor 25 of the line 2| is slightly less than that of the outer conductor 21 of the line 23, so that the conductors 25 and 21 are closely adjacent to each other, throughout the overlapping portion but do not touch. The inner conductor 29 of the line 23 is hollow at least through the overlapping portion, having an inner diameter slightly greater than the diameter of the inner conductor 3| of the line 2|.

In the operation of the structure of Figure 4, the overlapping outer conductors 25 and 21 constitute a low impedence concentric line, opencircuited at its outer end 33. Owing to the impedance inversion characteristics of quarter wave lines, the open circuit at the point 33 is reflected as a short circuit at the point 35, thus effectively connecting together the outer conductors 25 and 21 at this point. Similarly, the inner conductors act as a low impedance quarter wave line opencircuited at the point 31, and presenting a shortcircuit at the point .39. Thus at the frequency for which the structure is designed, the inner conductors 29. and 3| are. eifectivelyconnected together at the point 39,

If the clearances between the overlapping portions of the conductors of the lines 2| and 23 are relatively small, the characteristic impedances of the quarter wave open-circulted lines are low, and the impedances presented at the points 35 and 39 are substantially zero over a fairly wide frequency range. The width of the band for which a structure like that of Figure 4 can be designed depends upon how close the tolerances may be maintained upon the diameters of the overlapping conductors Considerable improvement in the operation of devicessuch "as that illustrated in Figure 4 may be obtained by stag ering. the inner and outer overlaps; so that reactance at the point 35 is inverted to resonate with that at the point 39, and vice versa, as described in copending U. S. application Ser. No, 494,617, filed July 14, 1943 by D. W. Peterson andentitled Radio frequency rotating joint. However, the operating band width is still limited by the minimum clearances which are practical. t

In accordancewith the present invention, a wider usable frequency range may be obtained with a given clearance than with the above described structures, or the same frequency ranges may be securedwith considerably greater clearance. Referring to Figure 5, two concentric line sections I2I and I23 are arranged so that their inner conductors and outer conductors are respectively overlapping by wavelength at the mean operating frequency, as in the system of Figure 4. However, theoverlapping portions are spaced apart longitudinally, so that the inner end I35 of the overlapping portion of the outer conductors I 25 and I21 is one-half wavelength distant from the end I39 of the overlapping portion of the inner conductors I29 and I3I. The final half wavelength portion I4I of the outer conductor I25 cooperates with the final half wavelength portion N3 of the inner conductor I29 to function as a half wavelength coaxial line section connected between the points I35 and I39. This section corresponds in function to the half wavelength line section 9 in the circuit of Figure 1. The quarter wavelength overlapping portion of the inner conductors |3| and I29 corresponds to the line section 1 of Figure 1. Similarly the quarter wave overlapping portion between the outer conductors I25 and I21 corresponds to the line I of Figure 1. I

The diameters of the conductors I25 and I3I are proportioned to provide a characteristic impedance Z0 equal to that of the remainder of the transmission circuit (not shown). The diameters of the conductors, I29 and I21 are similarly proportioned to provide a characteristic impedance Z0. The clearances between the outer conductors I25 and I21 and the inner conductors I29 and I3I are made as small as conveniently practical. The innerdiameter of the half wave section MI and the outer dianieterofthe half wave section I43 are proportioned to provide the proper characteristic impedance Zt in accordance with the information illustrated graphically in Figure 3. The device of Figure 5 will operate throughout, a relatively wide band of frequencies without introducing substantial reflection,

Although the invention has been described as embodied in a rotatable joint for coaxial trans: mission lines, it is not limited thereto, but may be similarly applied in substantially any case where an undesirable series-reactanee is to be compensated. Briefly, the invention comprises the connection of ahalf wavelength transmission line tothe point at which the reactance to be compensated appears. A reactanoe element substantially identical with that to be cornpen= sated is connected inseries with the half wavelength line section at its other end. By adjusta ing the characteristic. impedance of the half wavelength line in accordance with that of the reactance elements and the impedance of the transmission circuit, compensation is effected throughout a relatively wide band of frequencies.

;; clai nas my invention: 1 I

1. In a radio frequency netwgrl; including a load of impedance Z0, in series with an element which exhibits series resonant characteristics'to energy of wavelength A, and has a surge impedance Z5, a system lior compensating the reactance of said element throughout a band of frequencies, comprising a transmission line of length A/2 and surge impedance Zc, connected at one of its ends to said load, and a second element exhibiting series resonant characteristics identical with those of said first-mentioned element, connected in series with the other end of said line, the surge impedance Zc of said line being such as to-satisfy the relation:

2. In a radio frequency network including a source of impedance Z0, in'series with an element which exhibits series resonant characteristics to energy of Wavelength A, and has a surge impedance ZS, a system for compensating the reactance of said element throughout a band of frequencies, comprising a transmission line of length X/2 and surge impedance Zc, connected at one of its ends to said source, and a second element exhibiting series resonant characteristics identical with those of said first-mentioned element, connected in series with the other end of said line, surge impedance Zc of said line being such as to satisfy the relation:

3. In a radio frequency network including a load of impedance Zn, in series with an element which exhibits series resonant characteristics to energy of wavelength A, and has a surge impedance ZS, a system for compensating the reactance of said element throughout a band of frequencies, comprising a transmission line of length A/2 and surge impedance Zc, connected at one of its ends to said load, and a second element exhibiting series resonant characteristics identical with those ofsaid first-mentioned element, connected in series with the other end of said line, the surge impedance Zc of said line being such that the quantity has a value lying within the range of 90 percent to 100 percent of that of the quantity:

Z 2 1+ /1+4( *T a l. In a radio frequency network including a source of impedance Zn, in series with an element which exhibits series resonant characteristics to energy of wavelength A, and has a surge impedance ZS, a system for compensating the reactance of said element throughout a band of frequencies, comprising a transmission line of length A/2 and surge impedance Zc, connected at one of its ends to said source, and a second eiement exhibiting-series resonant characteristics identical with those of said first-mentioned element, connected in series with the other end of said line, the surge impedance Zc of said line being such that the quantity has a value lying within the range of 90 percent to 100 percent of that of the quantity:

5. A joint for connecting together two relatively rotatable coaxial transmission lines, comprising a pair of concentric tubular conductive members disposed in radially spaced overlapping relationship to each other and connected to the respectiveouter conductors of said two coaxial lines, a second pair of concentric conductive members disposed coaxially within said first pair of conductive members and in radially spaced overlapping relationship to each other, the end of the inner of ,said.first two conductive members being spaced longitudinally along the axis of said lines a distance of substantially A/2 from the end of the outer of said second pair of conductive members, where A is the mean operating wavelength of the system, and the ratio of the inner diameter of said-inner conductor of said first pair to the outer diameter of the outer conductor of said second pair being less than the ratio of the diameters of the outer conductors to the diameters oi the inner conductors of said 'of conductive members and in radially spaced 'system, and the ratioof the innerdiameter of said inner conductor of said first pair to the outer diameter of the outer conductor of said second pair being less than the'ra'tio of the diameters of the outer conductors to the diameters of the inner conductors of'said'coaxial lines.

'7. A joint for connecting together two relatively rotatable coaxial transmission lines, comprising a pair of concentric tubular conductive members disposed in radially spaced overlapping relationship to each other and connected to the respective outer conductors ofsaid two coaxial lines, a second pair of concentric conductive members disposed coaxially within said first pair of conductive members and in radially spaced overlapping relationship to each other, the end of the inner ofsaid first two conductive members being spaced longitudinally along the axis of said lines a distance of substantially A/2 from the end of the outer of-said second pair of conductive members, where A is the-mean operating wavelength of the system, and the ratio of the inner diameter of said inner conductorfof said first pair to the outerdiameter of the outer con- 9 duct-or of said second pair being such that the characteristic impedance 20 between said lastmentioned two conductors is related to the characteristic impedance ZS between the conductors of each of said pairs and the characteristic impedance Z of said transmission lines so that the quantity is within the range of 90 percent to 100 percent of the quantity:

8. A joint for connecting together two relatively rotatable coaxial transmission lines, comprising a pair of concentric tubular conductive members disposed in radially spaced relationship to each other overlapping through a distance of substantially A/ l and connected to the respective outer conductors of said two coaxial lines, a second pair of concentric conductive members disposed coaxially within said first pair of conductive members and in radially spaced relationship to each other overlapping through a distance of substantially M4, the end of the inner of said first two conductive members being spaced longitudi- 10 nally along the axis of said lines a distance of substantially M2 from the end of the outer of said second pair of conductive members, where A is the mean operating wavelength of the system, and

v the ratio of the inner diameter of said inner conductor on? said first pair to the outer diameter of the outer conductor of said second pair being such that the characteristic impedance Zc between said last two conductors is related to the characteristic impedance Zs between the conductors of each of said pairs and the characteristic impedance Z0 of said transmission lines substantially as follows:

2 w e) "T GEORGE H. BROWN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,147,807 Alford Feb. 21, 1939 2,270,416 Cork et a1 Jan. 20, 1942 2,274,346 Roosenstein Feb. 24, 1942 

